The invention relates to a method of anodizing aluminum capacitor foil.
So-called xe2x80x9csolidxe2x80x9d tantalum capacitors which employ pyrolytically deposited manganese dioxide as the cathode material readily lend themselves to surface mount applications and have become the device of choice for circuit designers desiring the maximum capacitance in a given case size, high reliability, and high resistance to the temperatures associated with the reflow soldering techniques used to attach surface mount components to circuit boards. Aluminum electrolytic capacitors containing a liquid electrolyte may be fabricated with a base which facilitates surface mounting under carefully controlled solder reflow conditions, but these devices lack the resistance to high solder reflow temperatures and the parametric stability over a broad temperature range characteristic of their tantalum surface mount counterparts.
In recent years, tantalum and aluminum capacitors containing intrinsically conductive polymer cathode materials, such as polypyrole, polyaniline, polythiophene, and derivatives thereof, have been introduced commercially. The high electrical conductivity and thermal stability of certain of these intrinsically conductive polymers makes them almost ideally suitable for use as cathode materials for surface mount electrolytic capacitors. The introduction of these materials has led to a reduction in device ESR and reduced ignition damage from shorted capacitors as well as to the introduction of truly surface mountable aluminum electrolytic capacitors.
Unfortunately, aluminum electrolytic capacitors which contain intrinsically conductive polymer cathode materials, and which are not hermetically sealed against contact with the atmosphere, tend to undergo degradation of their leakage current performance with time, particularly when exposed to a humid environment. The tendency for the leakage current of conductive polymer containing, surface mount aluminum capacitors to increase with time has been traced in large part to the tendency of the anodic aluminum oxide to react with the humidity in the atmosphere to form hydrated aluminum oxide which is known to have very poor properties as an insulating dielectric film. In xe2x80x9cwetxe2x80x9d aluminum capacitors, which contain organic liquid electrolyte solutions, hydration degradation of the aluminum oxide film is minimized by limiting the water content of the electrolyte solution and by including a small amount of orthophosphate ion in the electrolyte composition. Orthophosphate, adsorbed on the anodic oxide surfaces, acts as a barrier layer, which inhibits the hydration reaction.
In surface mount devices containing conductive polymer cathode materials, the moisture of the atmosphere has ready access to the anodic oxide film due to the absence of a hermetic seal and the relatively porous nature of the usual conductive polymer cathode materials. Destructive hydration of the anodic oxide in polymer cathode aluminum capacitors is inhibited by the presence of an adsorbed layer of phosphate applied after the anodizing step(s) and prior to the application polymer cathode material. Unfortunately, the relatively thin coating of phosphate adsorbed on the outer surface of the anodic oxide is easily damaged during assembly of the capacitors. Any break or crack in the outer phosphate layer allows atmospheric moisture access to the reactive anodic oxide film and generally results in hydration damage and increased device leakage current.
Provisional Patent Application Serial No. 60/296,725 and co-pending U.S. application Ser. No. 09/891,208, now Pat. No. 6,450,565, describe the large hydration stability advantage obtained through the use of aluminum capacitor anode foil which has been anodized in a phosphate solution rather than in the dicarboxylic acid salt solutions normally used to anodize low voltage, high surface area capacitor foil. As described in this application, aluminum foil, which has been anodized in a phosphate solution has been found to be extremely resistant to hydration degradation even if the anodic oxide coating is cracked or damaged during the assembly of the finished capacitors.
Unfortunately, aqueous solutions containing orthophosphate as the sole anionic species tend to have a solvent action upon the aluminum foil during anodizing; this aluminum dissolution occurs even at near-neutral pH. The dissolved aluminum tends to form an aluminum phosphate precipitate which rapidly clouds the anodizing solution and which precipitates upon the anodizing tank surfaces as well as upon the foil being anodized. With the aluminum foil used for xe2x80x9cphase shiftingxe2x80x9d or xe2x80x9cmotor startxe2x80x9d capacitors, which has relatively shallow, wide diameter etch tunnels, the above described precipitation of aluminum phosphate upon the surface of the foil being anodized in a phosphate solution is of little consequence and aqueous, phosphate-based anodizing electrolyte solutions were used to anodize this type of foil for many years. With modern, highly etched aluminum anode foils having deep, closely packed, narrow diameter etch tunnels, aqueous phosphate anodizing tends to be even more corrosive towards the foil; the fine etch tunnel structure of modern aluminum capacitor foils may be partially or completely destroyed or blocked by precipitates during aqueous phosphate anodizing. Thus, modern, high surface area anode foil is not anodized in phosphate-containing electrolyte solutions.
In co-pending application Ser. No. 09/709,742, now Pat. No. 6,409,905, the use of glycerine-containing phosphate solutions for anodizing aluminum capacitor foils are described. Aluminum dissolution during anodizing may be reduced or eliminated through the use of glycerine-containing phosphate solutions rather than the use of aqueous phosphate solutions containing no glycerine. The nearly complete absence of aluminum dissolution in phosphate solutions containing an appropriate amount of glycerine facilitates the use of these solutions for the anodizing of modern, highly-etched foils. The hydration resistance advantages obtained with surface mount aluminum electrolytic capacitors, containing conductive polymer cathode materials and phosphate-anodized anode foil, have already been described.
The invention is directed to a process for anodizing aluminum foil comprising anodizing the foil in a first aqueous electrolyte solution, heating the foil in an oven, and anodizing the foil in a second anodizing solution, wherein the first aqueous electrolyte solution and second aqueous electrolyte solution each comprise about 5 wt % to about 50 wt % glycerine, about 0.01 wt % to about 0.2 wt % ammonium phosphate, and water, and wherein the foil is anodized in the first aqueous electrolyte solution for at least about 3.5 minutes. Preferably, the water is de-ionized.
In a preferred embodiment, the first and second aqueous electrolyte solutions each comprise about 5 wt % to about 25 wt % glycerine. In another preferred embodiment, about 0.1% ammonium phosphate is present in the first and/or second aqueous electrolytic solutions.
In a further preferred embodiment, the temperature of the first aqueous electrolyte solution and second aqueous electrolyte solution are each between about 80xc2x0 C. and 90xc2x0 C.
The present invention is directed to a process for the continuous anodizing of aluminum foil for use in aluminum electrolytic capacitors. Specifically, etched anode foil is anodized to relatively low voltage in a two-step reel-to-reel process. The present invention is particularly useful for anodizing highly-etched aluminum foil for use in surface mount aluminum capacitors containing conductive polymer cathode material. The process of the invention is economical and provides high foil quality.
The process of continuous anodizing of aluminum foil via passing the foil through tanks containing anodizing electrolyte solution in a xe2x80x9creel-to-reelxe2x80x9d process has been known for many years and is described in Paul M Deeley, Electrolytic Capacitors Comell-Dubilier Electric Corporation, South Planefield, N.J., 1938, pp 105-118. The main improvement since Deeley was published is the use of near-neutral pH solutions of the ammonium salts of dicarboxylic acids such as ammonium adipate in place of the boric acid/borax solutions employed in the 1930""s. Carboxylic acid salt solutions result in very high anodizing efficiency with minimal aluminum dissolution and are well suited to the anodizing of high surface area (i.e., highly etched) aluminum foil.
It was discovered that a process similar to Deeley""s reel-to-reel anodizing process can be used to anodize highly tunnel-etched aluminum foil to produce a highly hydration-resistant oxide film on the aluminum. This process utilizes a phosphate ionogen electrolyte solution containing glycerine (xe2x80x9cglycerolxe2x80x9d), which inhibits the aluminum dissolution reaction. This electrolytic solution is described in co-pending Application Ser. No. 09/709,742, which is hereby incorporated by reference in its entirety.
It was further discovered that the residence time which the foil spends exposed to voltage has a direct influence upon the quality of the oxide produced from a leakage current standpoint. There is a rapid improvement in leakage current between a residence time at voltage before heat-treatment of 1 minute and 4 minutes. The leakage current performance of the foil improves above a residence time at voltage of 4 minutes, but much more slowly than before a residence time of about 4 minutes is reached.
The process of the invention may use multiple anodizing tanks, such as are depicted schematically on page 111 of Deeley, or one large anodizing tank. The foil may pass through a suitable oven, such as a tunnel oven, between anodizing tanks, approximately halfway through the anodizing process. The foil is heat treated in the oven.
In accordance with one embodiment of the invention, the foil is immersed in the aqueous electrolyte solution and anodized (first anodizing step), then the foil is passed through the oven (heat treatment step), and then the foil is immersed in a second aqueous electrolyte solution (second anodizing step).
The aqueous electrolyte solution has a relatively low concentration of glycerine, e.g., about 5 wt % to about 50 wt %, preferably about 5 wt % to about 25 wt %, more preferably about 10 wt % to about 20 wt % glycerine, in water containing a relatively low amount, e.g., about 0.01 wt % to about 0.2 wt %, preferably about 0.1 wt %, of ammonium phosphate. Preferably, the water is de-ionized. The temperature of the solution is maintained between about 80xc2x0 C. and 90xc2x0 C.
The residence time of the foil in the aqueous electrolyte during the 1st anodizing step is at least 3.5 minutes, preferably at least about 4 minutes, more preferably between about 4 minutes and 60 minutes. Typically 15 minutes provides the desired anodization results.
The applied voltage is typically about 4 to about 100, typically about 4 to about 50.
A rinse step, typically with water, may be employed between the first anodizing step and the heat-treatment step. A rinse step is likely unnecessary due to the nearly complete evaporation of the aqueous glycerine/ammonium phosphate solution from the foil as the foil is heated in the in-line tunnel oven.
The temperature of the oven is typically between about 300xc2x0 C. and about 550xc2x0 C., preferably between about 350xc2x0 C. and about 500xc2x0 C., and most preferably between about 400xc2x0 C. and about 450xc2x0 C. Preferably, sufficient air should flow through the oven to prevent build-up of electrolyte decomposition products on the internal surfaces of the oven during use. Typically the residence time is about 5 seconds to about 5 minutes.
The foil is then anodized at least one more time at approximately the same voltage, electrolyte composition, and temperature as used in the first anodizing step, although the electrolyte may contain a lower concentration of ammonium phosphate and exhibit a higher resistivity if desired in order to facilitate drying at a lower temperature.
The foil may then be dried with room temperature or heated air, by passing through a tunnel oven or any other suitable oven. Other gases, such as nitrogen, argon, oxygen, or carbon dioxide, could be used in place of air. The foil may be rinsed before final drying, but this is not necessary if sufficiently vigorous drying methods are employed. The anodized foil may then be wound onto a take-up reel in preparation for further processing into surface mount capacitors.
The effects of both the heat-treatment and residence time at voltage prior to heat-treatment are illustrated in the following example.